Allow filtering of login message to allow for custom messages to be shown on the login page

[DD32]

Long story short, I'm wanting to be able to display a message on the login form, regardless of if the user is attempting to login, register, retrieve their password, or whatever else they may do.

But the problem is, All the filters available on the login form currently only filter the messages if there is a message to display in the first place.

The attached patch moves the filter login_message to always be run on the login page, even when no $message is passed to the login_header()

An example of the use of this would be something as such:

add_filter('login_message', 'theme__login_message');
function theme_login_message($message) {
	return $message . '<p class="message"><a href="/wiki/register-info">For more information on the benefits of Registering, Please see our wiki page.</a></p>';
}

[filosofo]

I was just about to create the exact patch, but I found this ticket. Let's get this into 2.8, please.

[ryan]

(In [10634]) Allow filtering of login message even if empty. Props DD32. fixes #8675